Boron is an essential plant micronutrient that is toxic at high soil concentrations. The range between boron deficiency and toxicity is narrow for plants, and imbalances in boron nutrition are common in agriculture. Whilst deficiency may be addressed easily through the application of boron rich fertilisers, boron toxicity is more difficult to manage agronomically. Boron levels are generally higher in subsoils than the surface root zone, making amelioration through soil modification difficult and impractical.
Boron toxicity is a major limitation to cereal production in southern Australia, and is also a problem in arid and semi-arid parts of the world including Asia and Africa. Yield penalties of up to 17% between adjacent areas of barley have been attributed to differences in shoot boron concentration. Similar figures (11%) have been reported for wheat in southern Australia. Soils high in boron tend to be associated with low rainfall environments (250-450 mm per year) and primarily derived from clay rich sediments of marine origin. Increasingly, boron toxicity is becoming associated with irrigated environments, where groundwater application contributes to an excessive accumulation of boron. Frequently boron is found at high concentrations in saline soils.
The known function of boron in plants is as a structural and functional component of cells walls and the plasma membrane. In plants boron exists primarily as boric acid [B(OH)3], and to a lesser extent at neutral and alkaline pH as the borate anion [B(OH)4−]. Under adequate boron supply, uptake from the soil into plant roots via the plasma membrane is a passive process, and one that occurs rapidly: The half-time of influx into barley roots is approximately six minutes. Inside the plant cell, the ability of boron to form stable complexes with hydroxy compounds has been studied extensively. Examples of molecules that form complexes with boric acid include ribose, apiose, sorbitol and other polyols, glycoproteins and glycolipids. The binding of boron to apiose, the central component of the rhamnogalacturonan-II complex in primary plant cell walls is needed to maintain the normal physical properties of cell walls.
In vascular plants, boron moves from the roots within the transpiration stream and accumulates at the tips of older leaves. A sharp concentration gradient is observed within the leaf, and toxicity symptoms are directly correlated with boron distribution. Symptoms appear first at the tips of older leaves, where a high boron concentration leads to chlorosis and necrosis, first extending down the leaf margins. In barley this generally occurs at cellular concentrations in excess of 23 mM.
Variation in tolerance to boron toxicity exists both between and within species. The primary mechanism of tolerance appears to be similar for all species studied: an ability to maintain low concentrations of boron in plant tissues. However, the molecular basis for this is currently unknown. Boron tolerant genotypes generally accumulate lower concentrations of boron than intolerant genotypes, suggesting that exclusion rather than internal tolerance mechanisms are operating. It is however likely that other mechanisms related to internal tissue tolerance to boron are present in plants and have a significant role.
Interval regression mapping in both wheat and barley has identified the chromosomal location of several QTL for various boron tolerance traits. In barley, four QTL (on 2HS, 3HS, 4HL, 6HL) have been identified that have detectable effects on boron tolerance in barley. These are: Leaf Symptom Expression (a measure of severity of symptoms on the basis of leaf damage), Relative Root Length (root length at boron 100 mg/L−1 expressed as a percentage of the root length at boron 0 mg/L−1), Whole-shoot boron Concentration (shoot boron concentration of 5 week old plants as measured by Inductively Coupled Plasma Spectrometry, ICP) and Dry Matter Production (dry weight).
TABLE 1Percentage variation associated with boron tolerance QTL:PercentageParameterLocitrait variationLeaf Symptom Expression2HS, 4HL38Relative Root Length3HS, 4HL39Whole-shoot Boron Concentration4HL, 6HL53Dry Matter Production4HL34
It would be desirable to identify the nucleotide and amino acid sequences which encode boron transporters, and a way in which these could be expressed in plant tissues to avoid boron toxicity. The identification of such sequences would allow, among other things, the introduction, removal or modulation of boron transport activity in a range of cells and/or organisms, including plant cells and plants.
Reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in any country.